PRACTICAL CONCLUSIONS 295 



Another great source of infection is in all probability the 

 milk of cows affected with tuberculosis of the udder. In such 

 cases animal inoculation with centrifugalised samples of the milk 

 is the only reliable means of detecting the presence of tubercle 

 bacilli. As pointed out by Woodhead and others, the milk 

 from cows thus affected is probably the great source of tabes 

 mesenterica, which is so common in young subjects (vide 

 p. 287). In these cases there may be tubercular ulceration of 

 the intestine, or it may be absent. Woodhead found that out 

 of 127 cases of tuberculosis in children, the niesenteric glands 

 showed tubercular affection in 100, and that there was ulceration 

 of the intestine in 43. It is especially in children that this 

 mode of infection occurs, as in the adult ulceration of the intes- 

 tine is rare as a primary infection, though it is common in 

 phthisical patients as the result of infection by the bacilli in the 

 sputum which has been swallowed. There is less risk of infec- 

 tion by means of the flesh of tubercular animals, for, in the first 

 place, tuberculosis of the muscles of oxen being very rare, there 

 is little chance of the bacilli being present in the flesh unless the 

 surface has been smeared with the juice of the tubercular organs, 

 as in the process of cutting up the parts ; and, in the second 

 place, even when present they will be destroyed if the meat is 

 thoroughly cooked. 



We may state, therefore, that the two great modes of infection 

 are by inhalation, and by ingestion, of tubercle bacilli. In 

 the former, the tubercle bacilli will in most cases be derived 

 from the human subject ; in the latter, probably from tubercular 

 cows, though inhaled tubercle bacilli may also be swallowed and 

 contamination of food by tubercular material from the human 

 subject may occur. Alike when inhaled and when ingested, 

 tubercle bacilli may lodge about the pharynx and thus come to 

 infect the pharyngeal lymphoid tissue, tonsils, etc., tubercular 

 lesions of these parts being much more frequent than was 

 formerly supposed. Thence the cervical lymphatic glands may 

 become infected, and afterwards other groups of glands, bones, 

 or joints, and internal organs. 



The Specific Eeactions of Tubercle Bacilli. The tubercle 

 bacillus belongs to the group of organisms which do not to any 

 extent secrete soluble toxins, but which nevertheless produce 

 effects in the body at a distance from the site of actual prolifera- 

 tion. The origin of these effects is obscure, but there is abundant 

 evidence that, while the injection of dead bacilli tends to pro- 

 duce local lesions, the introduction of the disintegrated proto- 

 plasm of the bacillus can produce pathogenic effects of a toxic 



